Incorporated herein by reference is the disclosure of copending U.S. patent application Ser. No. 08/115,810, filed Sep. 3, 1993, and entitled "Video Merging Employing Pattern-key Insertion", which is assigned to the same assignee as the present application. As taught in that patent application, pattern-key insertion is used to derive a composite image by merging foreground and background. The implementation techniques used for this purpose is one in which an estimate of the location of a target region can be inferred from the tracked location of any of multiple landmark regions in the background image. The location of each of the multiple landmark regions may be displaced in a different direction from the location of the target region, so that in case the video scene is such that the target region itself moves partially or completely beyond a particular edge of the image, at least one of the tracked multiple landmark regions remains within the image so that even if the location of the target region itself is partially or wholly outside of the image field of view, inferred tracking of the target region itself can still be continuously maintained. In addition, any of the tracked multiple landmark regions in the image may be occluded at times by the presence of a foreground object in the scene, so it cannot be used at such times for inferring the location of the target region. In such a case, another of the tracked multiple landmark regions in the image must be used instead. However, it has been found that switching from one tracked multiple landmark region to another tracked multiple landmark region for use in inferring the location of the target pattern results in model errors that cause unstable estimates of the location of the target pattern.
Such model errors could be reduced by fitting higher order models to the respective tracked multiple landmark regions so that they are tracked better. Such higher order models are unstable to estimate from a single image frame, and biased errors in local estimates introduce estimation errors that are difficult to model a priori.
Further incorporated herein by reference is the disclosure of copending U.S. patent application Ser. No. 08/222,207, filed Mar. 31, 1994, and entitled "Stabilizing Estimate of Location of Target Region Inferred from Tracked Multiple Landmark Regions of a Video Image", which is also assigned to the same assignee as the present application. Taught in that patent application is an efficient method for performing stable video insertion of a target pattern even when different ones of multiple landmark regions are tracked at different time intervals for use in inferring the location of the target region from the location of that particular one of the multiple landmark regions then being tracked. Specifically, due to occlusion or disocclusion by foreground objects, or disappearance or appearance as the camera pans and zooms across a scene, the tracking landmark region is switched from one of the multiple landmark regions to another. This works well only when landmark regions are visible, are unchanging over time, and when the model relating the current image to the reference image fits accurately.